Rough idle, backfire on acceleration

1. Worn clutch: If the clutch is worn, it may not be disengaging properly when you change gears, causing the engine to continue to rev as if you were still accelerating. This can also cause difficulty shifting gears or a slipping feeling when accelerating.
2. Dirty or faulty throttle body: The throttle body controls the amount of air entering the engine and adjusts the idle speed. If it's dirty or faulty, it may not be closing properly after you release the accelerator pedal, causing the engine to continue to rev.
3. Vacuum leak: A vacuum leak can cause the engine to rev excessively or idle roughly. The leak can be caused by a cracked or disconnected vacuum hose, or a malfunctioning intake manifold gasket.
4. Faulty idle air control valve: The idle air control valve regulates the engine's idle speed. If it's faulty, it may not be closing properly after you release the accelerator pedal, causing the engine to continue to rev.

SYMPTOM 1: ROUGH IDLE IN THE COLD

The engine will not idle smoothly, or it stalls during idle when the engine is cold. When the engine is cold and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine, or at least it runs more smoothly.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace accelerator pump or replace the carburetor.
2. There may be a vacuum leak.
   The Fix: Check and replace the vacuum lines as required.
3. There may be some type of ignition problem.
   The Fix: Check and replace the distributor cap, rotor, ignition wires, and spark plugs.
4. The ignition timing may be set wrong.
   The Fix: Adjust the ignition timing.
5. There may be a fault in the computerized engine control system.
   The Fix: Check engine control systems with a scan tool. Test the circuits and repair or replace components as required.

1. The EGR valve may be bad.
   The Fix: Replace the EGR valve.
2. The engine may have mechanical problems.
   The Fix: Check the compression to determine the engine's condition.
3. Idle speed is set incorrectly.
   The Fix: Set idle speed to the car's original settings.
4. The fuel injectors may be dirty.
   The Fix: Clean or replace the fuel injectors.

SYMPTOM 2: ROUGH IDLE WITH WARM ENGINE

The engine will not idle smoothly, or it stalls during idle when the engine is warm. When the engine is warm or hot and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace accelerator pump or replace the carburetor.
2. There may be a vacuum leak.
   The Fix: Check and replace vacuum lines as required.
3. The fuel pressure regulator may be operating at too low a pressure.
   The Fix: Check the fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job.)
4. Idle speed set incorrectly.
   The Fix: Set idle speed to specs.
5. There may be some type of ignition problem.
   The Fix: Check and replace distributor cap, rotor, ignition wires and spark plugs.
6. There may be a fault in the computerized engine control system.
   The Fix: Check engine control systems with a scan tool. Test circuits and repair or replace components as required. (This is not really a do-it-yourself kind of job.)
7. The EGR valve may be bad.
   The Fix: Replace EGR valve.

1. The engine may have mechanical problems.
   The Fix: Check compression to determine engine condition.
2. The fuel injectors may be dirty.
   The Fix: Clean or replace fuel injectors.

SYMPTOM 3: FAST IDLING

The engine idles too fast. After the engine has run long enough to become warm, the idle speed does not come down to normal. You really notice it when you come to a stop and must have to push ******* the brake pedal to keep the car from moving.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace the accelerator pump or replace the carburetor.
2. The engine may be overheating.
   The Fix: Check and repair the cooling system.
3. The fuel pressure regulator may be operating at too low a pressure.
   The Fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job.)

1. The ignition timing may be set wrong.
   The Fix: Adjust the ignition timing.
2. There may be some type of ignition problem.
   The Fix: Check and replace the distributor cap, rotor, ignition wires and spark plugs.
3. There may be a fault in the computerized engine control system.
   The Fix: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required.
4. There may be a vacuum leak.
   The Fix: Check and replace vacuum lines as required.
5. You have a bad idle speed control unit.
   The Fix: Replace the idle speed control unit.
6. The alternator may not be working properly.
   The Fix: Replace the alternator.

SYMPTOM 4: STALLING UPON STOPPING

Car stalls when stopped quickly. You are driving along and everything is just fine ... until you let off the gas pedal and apply the brakes. The engine starts shaking and may even stall. Not a good thing to happen because you lose power steering when the engine dies and could risk an accident.
Possible causes:

1. There may be a serious vacuum leak.
   The Fix: Check and replace the vacuum lines as required.
2. There may be a fault in the computerized engine control system.
   The Fix: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required. (This is not really a do-it-yourself kind of job.)
3. Broken linkage.
   The Fix: Repair or replace the linkage as required.

Idling issues can be very frustrating, but with some patient troubleshooting, you'll have a real chance at figuring it out. Remember to always check your engine idle with the air conditioning and defroster turned off, as both of these systems are designed to change the idle when they are on due to the air conditioning system's demands on the engine.

Rough Running Engine - James on Engines #2 - Bell Performance

https://www.bellperformance.com/blog/rough-running-engine

Apr 21, 2015 - When diagnosing an engine with a rough idle, there are certain things that must be ... If the problem does not reoccur, the codes will be erased by the computer. ... when accelerating the vehicle, which causes a higher fuel demand. .... it was water in gas...added added Gas LineAntifreeze/Water Remover.

Check the fuel pressure, could be the fuel pump

I'd say sounds kinda like a missfire although with the absence of an engine code I would lean more towards a vacuum leak somewhere. Chk all your lines while cars running to see if you might have a vacuum leak.

Does it idle rough or have a low idle went the engine is hot? Check the fuel pressure regulator on the drivers side of the engine. It will be about in the middle of the fuel rail. It will have a small vacuum hose attached to it. Remove the hose and see if gas comes out of the regulator. If it does replace the regulator. If the regulator is dry than check for vacuum leaks around the intake plenum.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Really sounds like a vacuum leak. Lean code could still be accurate. Screaming idle usually means air is getting into engine. Ruptured EGR diaphragm would vent air. Brake booster could vent air. A lot of air will show lean mix.

You may have to use different methods to test vacuum. Some use vacuum gauge to test egr and lines which dead end into solenoids. Other tests use water spray or propane gas to draw in and change rpm. You are at a disadvantage if engine does not run.

Crankshaft sensor helps set fuel volume for given rpm. Might not be counting all the rpm's, but would not account for fast idle.

For this types of problem,there are few help inks to troubleshoot the problem.Click the link below to go through the troubleshooting procedure to resolve the issues :----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------van idles rough? http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html
--------------runs lean and idles rough? http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html
-----------Car still runs Rough after tune up? http://howtobyme.blogspot.com/2011/12/car-still-runs-rough-after-tune-up.html
------------Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed? http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html
----------High rpm in neutral? Inconsistant high RPM in neutral? http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html
------------Engine Backfires? http://technoanswers.blogspot.in/2012/02/jeep-backfires.html----------- Nissan stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

-----------
Nissan Pathfinder Backfires? http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-backfires.html
---------How to adjust rpm? http://schematicsdiagram.blogspot.com/2011/12/how-to-adjust-rpm-on-2009-dodge-charger.html
------------Engine cuts out at 3000 RPM? http://schematicsdiagram.blogspot.com/2011/12/engine-cuts-out-at-3000-rpm.html
----------This will help.Thanks.

Rough idle is a very common problem on 4 cylinders, but I believe the 98 Regency has a 6 cylinder in it, right?

Sometimes the coil packs get iffy, and sometimes the EGR Valve gets a leak in it causing a rough idle.

When the accelerator pedal is depressed, it can change things so that the engine seems to run smoothly.

However, I would look for codes to see if anything intermittent is occurring in the engine that may need attention.

The OBD-I scanner can help to diagnose the problem, if anything is out of specifications on the engine in question.

The owner will have to find a technician that has such a scanner.